High-precision temperature control system in crystal growth control

1 System Hardware Composition
　　The crystal growth temperature control system is shown in Figure 1. The high-precision temperature transmitter amplifies the detected weak temperature difference signal and converts it through A/D. The single-chip computer system performs data acquisition and analysis. On the one hand, the LED displays the temperature value collected on site. On the other hand, the collected signal is compared with the temperature value set by the keyboard to extract the temperature difference and the temperature difference change as the input parameters of the intelligent control. The output controls the thyristor drive circuit, and further controls the power of the heating rod to achieve the purpose of temperature control. Since crystal growth is carried out under rotation, the entire crystal carrier is controlled by a reversible motor. In addition, the system also designs a micro-printing interface and a temperature limit sound and light alarm circuit.
1.1 High-precision temperature transmitter
　　The system uses Pt100 as a temperature sensor. Its temperature coefficient α=0.00385/℃. For a small temperature difference of 0.001℃, the resistance value of Pt100 changes by about 0.385mΩ. After such a small resistance change is converted by the bridge, the maximum electrical signal can only reach 0.5~1μV. Therefore, the interface conditioning method for processing microvolt-level weak signals includes the design of high-precision unbalanced DC bridge, low-cutoff analog filter, low-noise, low-drift, high-sensitivity DC amplifier and grounding body.

　　From the circuit in Figure 2, we can see that
　　
　　
when the bridge is balanced, R1R4=R2R3, RT=R1+ΔR. Substituting the above formula and sorting it out, we get:
　　
　　For a specific temperature measurement and control system, R2, R3, and R4 are all known, VREF is the internal reference voltage of TL431, which is a constant value, so the output voltage ΔV of the bridge is linearly related to ΔR, that is, the bridge output is linearized. Resistors R2, R3, and R4 are all wire-wound precision resistors with small temperature coefficients and changing in the same direction, so that the bridge output signal achieves high stability.

　　AD524 is selected as the amplifier circuit of the temperature transmitter. Its gain can be adjusted by the external resistor RG. The temperature effect of RG will cause the AD524 gain factor to drift or the precision level to decrease. In the high-precision temperature control system, the temperature effect of the gain resistor must be compensated. The specific design method can be found in the literature [2].

　　The output signal amplified by AD524 is filtered out by the second-order low-pass filter connected to it to remove power supply interference. The passband width of the RC filter is designed to be 1.4Hz, which is suitable for the passband requirements of slowly changing temperature signals. For high-frequency interference signals, the low-pass filter has good anti-interference ability. In addition, the input signal is connected with twisted-pair shielded wires to reduce external electromagnetic interference. The input end of the amplifier adopts a tight symmetrical layout to reduce the influence of the contact thermocouple effect and improve the stability of the system.

1.2 Single-chip microcomputer system and its interface

　　The output signal of the temperature transmitter circuit is converted by the 16-bit A/D converter AD976 and analyzed and processed by the intelligent control system composed of 8031. The LED displays the on-site temperature value, and the output signal controls the working condition of the thyristor circuit, thereby controlling the operation of the heating rod in the tank to achieve the purpose of temperature control. The thyristor adopts zero-crossing triggering mode, and the output power adopts PWM pulse width regulation to avoid transient surge process of load current, reduce radio frequency interference and extend the service life of the thyristor.

　　In order to make the crystal grow evenly, the crystal carrier is required to be in rotation, that is, it is required to run continuously according to the law of forward rotation-stop-reverse rotation-stop-forward rotation. This process is realized by the reversible small motor and its control circuit in Figure 3. The system requires the motor to have a slow speed and a large torque. Therefore, a 10-watt ND-30 reversible motor is selected, in which C1 is the motor starting capacitor, and T1, T2, R, C2, and RW form a bidirectional thyristor motor speed control circuit. When 8031 ​​makes the P1 terminal output a low level and the P2 terminal output a high level, the solid-state relay SSR1 is closed and the motor rotates forward; when the P1 terminal outputs a high level and the P2 terminal outputs a low level, SSR2 is closed and the motor reverses. Its speed is achieved by adjusting R?W to control the conduction angle of the bidirectional thyristor T?1; when both the P1 terminal and the P2 terminal output a high level, the reversible motor stops rotating.

　　The system software consists of two parts: keyboard management system and intelligent controller. The keyboard management system provides functions including data setting, on-site temperature display, time display, restart, stop control, data printing, etc., providing a simple interface for human-computer interaction.

　　The intelligent control system realizes on-site data collection, intelligent control algorithms, and output control of the controlled process, and its principle block diagram is shown in Figure 4. The artificial crystal growth environment has the characteristics of large changes in the controlled environment, uncertain process curves, and difficulty in collecting real signals. Therefore, a two-level intelligent control strategy is adopted: the first level is the main control level, also known as the inner loop control, and the first level is the parameter correction control level, also known as the outer loop control. The comprehensive database is shared by the inner and outer loops, storing the prior knowledge of the controlled object, the required quality indicators, the prior values ​​of the control parameters, and the relevant dynamic values ​​of the system operation process, etc. It provides effective control data for the inner and outer loops.

　　The inner loop control process is also monitored by the outer loop. When the environment is relatively stable and the control effect of the main control level is good, the parameter correction level does not need to adjust the control parameters of the main control level. Once the controlled process or the user-set parameters change greatly and the control indicators of the main control level do not meet the user's requirements, the parameter correction level will be put into the adjustment process to improve the performance of the main control level by adjusting the control parameters of the main control level.

　　The core of the parameter correction level is also an intelligent controller. Its input parameters are the controlled field data, user settings, inner loop control conditions, etc. The controlled object is the control parameters of the main control level, and its variation range is small. After the input parameters are processed by data normalization, they are used as the reasoning conditions of the parameter correction reasoning system. According to the corresponding rules of parameter correction, the control parameters of the main control level in the comprehensive database are adjusted. This parameter adjustment process can also be carried out multiple times until the control effect of the main control level is improved, and the parameter correction stops adjusting the control parameters of the main control level. The rules used in parameter correction control are in the form of IFTHEN, and the rule set is also constructed according to the principle of "classification and stratification". The core of its reasoning system is also the heuristic subtree separation algorithm. The output of the reasoning system determines the adjustment of the inner loop control parameters, thereby improving the control quality of the main control level. The control parameters of the main control level can be dynamically adjusted (completed by the parameter correction level) in the system design, while the control parameters of the parameter correction level cannot be dynamically adjusted. ?

　　This temperature control system has been put into use by many units. Years of operation have shown that the temperature control accuracy is 0.01°C and the operation is stable and reliable. Table 1 is part of the test data of the temperature control process.

　　The use of this temperature control system can not only improve the temperature control quality, but also realize functions such as automatic temperature alarm, parameter tabulation, data printing, and information archiving, effectively improving the technology and management level of crystal production.